Furnace Annealing

 

 

The electrical behavior after implantation is dominated by deep-level electrons and hole-traps, which capture carriers and make resistivity high. Annealing is heating the wafer at high temperature for a specific time to repair lattice damage and to move dopant atoms on substitutional sites where they will be electrically active.

 

The success of the annealing is measured in terms of the fraction of the dopant atoms that will become electrically active.

 

Also the aim is to keep shallow implants shallow by minimizing diffusion.

 

Furnace annealing has times on the order of minutes. Annealing depends on dopant type, dose and whether the silicon has been made completely amorphous or if it is partially disordered.

 

For amorphous silicon, regrowth is by solid phase epitaxy. The amorphous/crystalline interface moves towards the surface at a fixed velocity that depends on temperature, doping and crystal orientation. Activation energy for SPE is 2.3 eV indicating that the process involves bond breaking at the interface. Impurities such as B,P, As increase the regrowth because they increase the number of broken bonds.[2]

 

If the silicon is partially amorphous then lattice repair occurs by the generation and diffusion of point defects. This process has an activation energy of about 5eV and requires temperatures on the order of 900°C to remove all defects.

 

Isochronal annealing plots, showing the fraction of activated dopant as a function of temperature for a fixed annealing time can summarize annealing characteristics.[1]

 

           

Fig. 8 Isochronal annealing of boron. The fraction of activated dopant is plotted against anneal temperature for different implant doses.[2]

 

 

Region I

Region II

Region III

Below 500°C point defects dominate free carrier concentration. As temperature increases these defects diffuse and combine. Net carrier concentration increases as many traps anneal out.

Above 500°C extended defects are formed which reduce the number of substitutional boron atoms and cause a net decrease in carrier concentration. This is called Reverse Annealing.

Above 600°C fraction of activated dopant atoms increases as point defect generation and migration allows precipitates and dislocations to dissolve.